Apparatus for polishing thin, flat semiconductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. Polishing heads of the type described above used in the CMP process are shown in U.S. Pat. No. 4,141,180 to Gill, Jr., et al.; U.S. Pat. No. 5,205,082 to Shendon et al; and, U.S. Pat. No. 5,643,061 to Jackson, et al. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by a non-uniform pressure applied on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
Referring now to FIG. 1C, wherein an improved CMP head 20, sometimes referred to as a Titan.RTM. head which differs from conventional CMP heads in two major respects is shown. First, the Titan.RTM. head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
More detailed views of the Titan.RTM. head are shown in FIGS. 2A and 2B. FIG. 2A shows that in a Titan.RTM. head, two separate pressure chambers of a carrier chamber 30 and a membrane chamber 32 are used during a polish process. A carrier pressure 34 exerts on the retaining ring 14, while a membrane pressure 18 translates into wafer backside pressure. The retaining pressure is a function of both the membrane pressure and the carrier pressure, for instance, P.sub.RR =2.039 P.sub.CAR -1.908 P.sub.MEM.
The operation of the Titan.RTM. head 20 can be shown in FIG. 2B. The Titan.RTM. head 20 picks up a wafer 10 by forming a suction cup with its membrane 16. A pressure is applied to the innertube 28 to force the membrane 16 downwardly onto the wafer 10 to ensure a good seal with the suction cup. A vacuum is thus applied to the membrane 16 to lift the wafer 10. The innertube 28 has little effect on the process because it is pressurized to the same pressure as the membrane chamber 32. During a polishing process, a pressure of approximately 5.2 psi is applied on the retaining ring which is higher than a pressure of approximately 4.5 psi that is applied on the membrane, i.e., on the wafer. The higher pressure applied on the retaining ring ensures that the wafer 10 is always retained in the retaining ring 14. However, after repeated usage, the bottom surface 36 of the retaining ring may be worn out and the wafer 10 may slide out during a polishing process. When such defective condition occurs, the wafer may be severely damaged or even broken.
FIG. 3 is a cross-sectional view of the continuation of an actual Titan.RTM. head. Within a Titan.RTM. head 20, three separate fluid chambers are utilized, i.e., a membrane chamber 38, an innertube chamber 40 and a retaining ring chamber 42. When a leakage occurs between either two of the three chambers, or between all three chambers, a "cross-talking" defect occurs which prevents either a pressure or a vacuum to reach its destination and causes a defective processing condition. For instance, when the vacuum is inadequate, the wafer may slip out and be scratched or broken. A leakage between chambers may further cause defects such as abnormal removal rate on the wafer surface or poor thickness uniformity across the entire wafer surface. It is therefore important that, before a chemical mechanical polishing process can be conducted, the three fluid chambers in the Titan.RTM. head be tested to detect any possible leakage between the chambers.
An enlarged, cross-sectional view of the membrane chamber 38, the membrane 16 and the membrane clamp 44 are shown in FIG. 4. When operating the Titan.RTM. head 20, if membrane 16 loses its elasticity, a dechuck sensor actuates to release the innertube pressure. The dechucking function therefore fails when the innertube pressure is released. When a membrane exceeds its lifetime (and therefore loses its elasticity), a serious defect of dechucking failure occurs. For instance, when the membrane exerts a -0.4 cm-Hg vacuum and the innertube exerts 1 psi pressure, the dechuck sensor should not be triggered to release the innertube pressure. However, when the membrane loses its elasticity it is no longer able to retain the required vacuum. The detection of a defective membrane, i.e., a membrane that has exceeded its lifetime, is therefore another important criterion in testing a Titan.RTM. polishing head before it is used in production.
An enlarged, cross-sectional view of the membrane 16 and the retaining ring 14, together with a carrier 46 and a retaining ring clamp screw 48 are shown in FIG. 5. When the membrane 16 is improperly mounted by the membrane clamp 44, i.e., for instance, a suitable distance or gap 50 between the membrane and the retaining ring 14 is not maintained, the membrane and the retaining ring may bind such that the membrane may be stuck with the retaining ring when exerting a downward pressure on a wafer. The binding that occurs between the membrane and the retaining ring is therefore another defect that should be detected before a Titan.RTM. head can be used in wafer processing.
In another partial, enlarged cross-sectional view of the Titan.RTM. head 20, shown in FIG. 6, an O-ring 52 and a spring 54 are incorporated in a dechucking sensor 56. When either the O-ring 52, the spring 54 or both are damaged or worn, a bad seal is resulted such that cross-talking occurs between the various fluid chambers 38, 40 and 42. It is desirable that, either the failure of the O-ring 52, the failure of the spring 54 or both should be detected to prevent a dechucking sensor failure prior to the use of a Titan.RTM. head in a CMP polishing process.
In a conventional Titan.RTM. head used for chemical mechanical polishing, it is impossible to verify the performance of the head without actually mounting the head in a CMP apparatus for a trial run. Such verification is frequently required when a rebuilt head is used, or when trouble shooting a defective head is desired. The down time of a CMP apparatus is increased as a result of the on-line testing. During such testing, wafer slipping out or broken on platen may also occur which contributes to more down-time. It is therefore desirable that if a rebuilt Titan.RTM. polishing head can be pre-tested before it is installed into a CMP apparatus for production use.
It is therefore an object of the present invention to provide a method for testing a polising head that does not have the drawbacks or shortcomings of the conventional test methods.
It is another object of the present invention to provide a method for off-line testing a polishing head for a CMP apparatus such that down time of the CMP apparatus can be avoided.
It is a further object of the present invention to provide a method for off-line testing a polishing head for a CMP apparatus by providing at least two sets of pressurizing/vacuuming/venting means capable of independently testing at least two fluid chambers in the polishing head.
It is another further object of the present invention to provide a method for off-line testing a polishing head for a CMP apparatus by providing three sets of pressurizing/vacuuming/venting means each adapted for testing a membrane chamber, an innertube chamber and a retaining ring chamber, respectively.
It is still another object of the present invention to provide a method for off-line testing a polishing head for a CMP apparatus that is capable of testing defects in the polishing head such as leakage between the fluid chambers, loss of seal in the fluid chambers and binding between the fluid chambers.
It is yet another object of the present invention to provide a method for off-line testing a polishing head for a CMP apparatus that can be used to qualify a rebuilt head prior to the installation of the rebuilt head into a CMP apparatus.
It is still another further object of the present invention to provide an off-line testing apparatus for a CMP head which includes at least two sets of pressurizing/vacuuming/venting means for testing a CMP head equipped with at least two fluid chambers.
It is yet another further object of the present invention to provide an off-line testing apparatus for a CMP head which includes three sets of pressurizing/vacuuming/venting means for testing a CMP head equipped with three fluid chambers of a membrane chamber, a retaining ring chamber and an innertube chamber.